There are large sex differences in the brain regions that control song in passerine birds. These sex differences develop under the control of sex steroid hormones. The long term objectives are to understand the mechanisms of sexual differentiation of brain and behavior, i.e., the cellular mechanisms by which sex steroid hormones influence neural development. The anticipated health benefit of the proposed studies is a better fundamental understanding of the synthesis of steroid hormones and their physiological effects on development of the brain and behavior. Current theories of sexual differentiation in zebra finches suggest that the male's brain synthesizes its own estrogen early in development, and this estrogen causes masculine development of the neural song system. Several methods will be used to find the sites of estrogen action during sexual differentiation: focal implants of sex steroids, lesions, and autoradiographic techniques to locate steroid receptors. The synthesis and metabolism of estrogen will be measured in homogenates of dissected regions of the brain to look for sex differences in the availability of estrogen during development. Various inhibitors of estrogen synthesis will be tested and used in an attempt to block the masculinization that occurs in males. In cultures of neurons and glia derived from various brain regions at various ages, the synthetic enzymes for estrogen and other steroids will be measured in males and females in order to look for sex differences. Estrogen receptors will also be measured in these cultures. Little is known about the role of sex steroid hormones during embryonic life, and there are good reasons to suspect that steroids act during this period to initiate the process of sexual differentiation. This idea will be tested by administering steroids or inhibitors of steroid synthesis to embryos to assess the effects of these treatments on development of brain and behavior.